The Royal Air Force has foiled a major Islamic State terror plot on the UK using autonomous electronic surveillance dronesRoyal Air Force

An Islamic State (Isis) terror plan to attack four UK cities in the wake of the Paris massacres was foiled thanks to the Royal Air Force (RAF), who intercepted conversations between two commercial pilots discussing potential targets over radio.

In 2015, RAF operators at the National Air Traffic Services (NATS) Control Centre in Hampshire picked up on two foreign commercial pilots discussing potential attacks on London, Bath, Brighton and Ipswich on the international commercial airline mayday radio channel, which the pilots believed was unmonitored. The pilots carried out their conversations in Arabic, and also used coded language that made reference to pop songs.

Pilots' conversations intercepted in the Netherlands

The messages were intercepted as the two pilots flew from Schiphol Airport in Amsterdam to destinations in the Middle East, and it is believed they were intending to smuggle chemical weapons and explosive devices into the UK, a senior RAF source told the Sunday Express.

RAF passed recordings of the conversations to GCHQ, where experts deciphered the calls in seven hours to discern that the attacks were planned to take place just after the Paris attacks in November 2015. The pilots made repeated references to chart "hits" to disguise the true nature of their conversation, while the phrase "climbing up the charts" referred to an increase in recruiting new Jihadists once the attacks had been launched.

"Alarm bells rang after several communications in code involving overseas airline pilots were picked up by chance. We can only assume that they considered it safer to use this frequency than other modes of communication," a senior RAF official said.

"We immediately passed these to the security services who then asked us to monitor certain airlines entering UK airspace."

Pilots and their airlines put onto aviation watchlists

UK authorities were not aware of the pilots prior to hearing them on the radio, and their voices were not a match for an existing database of potential terrorists, so is believed that these men are IS sympathisers.

The decision was made to let the two pilots continue to fly their commercial jets to the intended destinations without hindrance, and GCHQ now knows their identities and has put them onto a high-profile watchlist. The pilots were flying for two different commercial airlines, and it is believed that one of the airlines was already on a UK aviation watchlist.

The Express says that the foiled terror attack was one of the reasons behind Operation Templer, where David Cameron announced that up to 10,000 soldiers would be on standby to support police all over the UK in case a major terrorist attack occurs, but the Ministry of Defence told IBTimes UK that this plan exists to generally improve Britain's defence.

Regarding the foiled terror plot, a spokesperson for Her Majesty's Government said: "We do not comment on matters of national security."

The UK government has authorised the RAF to deploy autonomous remotely piloted electronic surveillance aircraft beyond UK airspace in order to monitor certain airlines, with support from the US Air Force at its bases in Germany. There is no confirmation either way as to how the messages were intercepted, but it could be possible that surveillance drones were used.